Chamonix
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787 Cabin Pressure

Fri Sep 09, 2011 1:49 pm

Does the 787 have the highest of all cabin pressures ever due to its bleedless air system?
The 787 will operate with a higher cabin pressure giving passengers the feeling of being at 6,000 feet above sea level rather than the 8,000 feet for other airliners.
 
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Starlionblue
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RE: 787 Cabin Pressure

Fri Sep 09, 2011 2:45 pm

I don't think the bleedless system per se enables higher pressure. The hurdles were likely more in fatigue life and such. Also the increased humidity means you have to watch more for corrosion.
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KELPkid
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RE: 787 Cabin Pressure

Fri Sep 09, 2011 3:06 pm

Quoting Starlionblue (Reply 1):
Also the increased humidity means you have to watch more for corrosion.

Only if you're using a material subject to corrosion to construct the fuselage   IIRC, there is no metal structure at all in the fuselage...100% composite. There is structural metal on the 787, just not in the fuselage.
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Stitch
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RE: 787 Cabin Pressure

Fri Sep 09, 2011 4:45 pm

Quoting Chamonix (Thread starter):
Does the 787 have the highest of all cabin pressures ever...

The A380-800 also offers a higher cabin pressure than commercial airlines currently in service, but I am not sure if it's ASL equivalent is equal to the 787's.
 
wingscrubber
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RE: 787 Cabin Pressure

Fri Sep 09, 2011 6:12 pm

All of the newer generation airliners will start to be designed to cope with the higher pressure differential caused by higher cabin pressure. On the 787 the electric air compressors of course have to be capable of matching what the bleed air system used to do, but as Starlionblue points out, it is the improvements in stress and fatigue life of the fuselage which make it possible.

Quick example

Sea level air pressure = 15 psi (ref)
40,000ft standard air pressure = 2.72 psi

8000ft = 10.64 psi
6000ft = 11.7 psi

So differential presure at 8000ft is 7.92psi, and at 6000ft it's 8.98psi. So only 1psi difference, but remember that's pounds per square inch - think about the internal surface area of an airliner, an extra pound of force for every sqaure inch of of the skin, that adds up to a lot of additional stress overall.
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rwessel
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RE: 787 Cabin Pressure

Fri Sep 09, 2011 6:51 pm

Quoting Chamonix (Thread starter):
Does the 787 have the highest of all cabin pressures ever due to its bleedless air system?
The 787 will operate with a higher cabin pressure giving passengers the feeling of being at 6,000 feet above sea level rather than the 8,000 feet for other airliners.

Several biz-jets allow higher cabin pressures. The G550, for example, has a hard limit of 10.48PSI, although is normally operated so that you get 6000ft in the cabin when at 51,000ft (which is 10.17PSI). It can normally maintain sea level cabin pressure up to 31,000ft (10.28PSI - which is the normal operating limit – and if you run that at 51,000ft, you get a 5500ft cabin altitude). The G550 should be able to maintain a lower cabin altitude than the 787 at any altitude the 787 can operate at (although neither aircraft will usually go below sea level).
 
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bikerthai
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RE: 787 Cabin Pressure

Fri Sep 09, 2011 7:07 pm

Quoting Starlionblue (Reply 1):
there is no metal structure at all in the fuselage...100% composite.

Technically the fasteners are metal.   

I'd be surprised if the "fork" fitting that attach the wing box to the fuselage is anything but metal. It could be a titanium fitting or strap which would reduce the corrosion issue. But it would be hard to avoid some metal in the structure. Specially at junction where large load transfer occurs.

bikerthai
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KELPkid
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RE: 787 Cabin Pressure

Fri Sep 09, 2011 7:57 pm

Quoting bikerthai (Reply 6):
I'd be surprised if the "fork" fitting that attach the wing box to the fuselage is anything but metal. It could be a titanium fitting or strap which would reduce the corrosion issue. But it would be hard to avoid some metal in the structure. Specially at junction where large load transfer occurs.

Yeah, but isn't that outside the pressure vessel, anyways?   Not susceptible to corrosion due to allowing humidity into the pressurization system (unless there's a poorly placed relief valve or something along those lines...).
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tdscanuck
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RE: 787 Cabin Pressure

Sat Sep 10, 2011 1:32 am

Quoting Chamonix (Thread starter):

Does the 787 have the highest of all cabin pressures ever due to its bleedless air system?

It's not because of the bleedless system. Any modern aircycle machine system runs at considerably higher pressure than you need to pressurize the cabin down to 6,000'. That's part of why pressure is maintained by regulating outflow, rather than inflow. All you need to do to raise the cabin altitude on a conventional bleed system is close the outflow valves. The problem is fuselage stress...CFRP is the enabler for the 787, not bleedless.

Quoting KELPkid (Reply 2):
IIRC, there is no metal structure at all in the fuselage...100% composite.

There's lots. Bulkheads, splice plates, seat tracks, and stanchions are all Ti. Still very limited corrosion issues though.

Tom.
 
pianos101
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RE: 787 Cabin Pressure

Tue Sep 20, 2011 4:20 am

Quoting tdscanuck (Reply 8):
Still very limited corrosion issues though.

Only with the Ti/CFRP interfaces. But remember, the 787 is still 20% Al and 15% Ti. The largest share (50%) is CFRP, but there is a lot more Al than you (not you Tom, but you know what I mean) would think. Corrosion is still a serious concern, since the 787 doesn't get sprayed all over with corrosion inhibiting compound like other models. There are specific design requirements in place to ensure that customer-promised maintenance schedules are adhered to and corrosion won't occur before those times.



Since Ti is 50% heavier that Al, it actually used sparingly, and only when the physical properties of the Ti are required. We won't make a metal fitting that fastens to CFRP out of Ti just for the sake of corrosion, since a fay seal is a perfectly adequate corrosion protection method.

Quoting Starlionblue (Reply 1):
Also the increased humidity means you have to watch more for corrosion.

Corrosion not as much as other things, such as moisture control. Higher humidity means more moisture content in the air, meaning more chances for water to collect in certain areas of the fuselage.
 
baroque
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RE: 787 Cabin Pressure

Tue Sep 20, 2011 9:43 am

And what will the relative humidity be in practice? On a long flight, finding all that water is not going to be all that easy????

I know I got bumped to a 744 on my last expected 380 flight but AFAIK the A380 is still in service so "than commercial airlines currently in service," should perhaps be "than other commercial airlines currently in service,"??     
 
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RE: 787 Cabin Pressure

Tue Sep 20, 2011 11:39 am

Quoting pianos101 (Reply 9):
Since Ti is 50% heavier that Al, it actually used sparingly, and only when the physical properties of the Ti are required.

I'm pretty sure that's not the case. Weight isn't the reason Ti is used sparingly since it's so much stronger than aluminum that you'd use a less of it volume wise. Ti is expensive and very hard to work with.
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dynamicsguy
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RE: 787 Cabin Pressure

Tue Sep 20, 2011 12:31 pm

Quoting nomadd22 (Reply 11):
Weight isn't the reason Ti is used sparingly since it's so much stronger than aluminum that you'd use a less of it volume wise.

That assumes that you design a part to be the same size regardless of the material. If you are not volume-limited then aluminium structure should be more weight efficient than Ti.
 
tdscanuck
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RE: 787 Cabin Pressure

Tue Sep 20, 2011 1:48 pm

Quoting Baroque (Reply 10):
And what will the relative humidity be in practice?

Depends on the operation...apples-to-apples (i.e. same flight duration and profile) it would be higher than a conventional aluminum airliner but the amount higher would depend on a lot of things. I don't know what the A380 humidity control system is like so I can't say anything intelligent there.

Tom.
 
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bikerthai
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RE: 787 Cabin Pressure

Tue Sep 20, 2011 3:55 pm

Isn't the relative humidity in the aircraft a red herring in the discussion of corrosion?

Even with the current humidity setting in the aircraft, you still get rain in the plane. Humidity, from the passenger breath will still freeze on the fuselage at altitude and melt when the airplanes lands, causing water drain down to the bilge. The problems still exist either way. So you have to deal with it.

Now with the graphite skin, you'll not have any skin corrosion. But any aluminum in the vicinity will need to be protected. They have been doing that on the Boeing and Airbus panes for a long time now (using serious methods) with varying results.

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KELPkid
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RE: 787 Cabin Pressure

Tue Sep 20, 2011 4:15 pm

Quoting tdscanuck (Reply 13):
Depends on the operation...apples-to-apples (i.e. same flight duration and profile) it would be higher than a conventional aluminum airliner but the amount higher would depend on a lot of things. I don't know what the A380 humidity control system is like so I can't say anything intelligent there.

Tom.

Doesn't the A380 use standard air packs (like just about every other jet transport)? I thought the air machine cycle naturally eliminates humidity through the course of its normal operation...
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Tod
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RE: 787 Cabin Pressure

Tue Sep 20, 2011 7:09 pm

Quoting Chamonix (Thread starter):
The 787 will operate with a higher cabin pressure giving passengers the feeling of being at 6,000 feet above sea level rather than the 8,000 feet for other airliners.

I usually carry an altimeter with me as a pax.

Typical 744 and 777 longhaul flights seem to range 5600 to 6500
Domestic 737, MD80, etc seem to typically run about 7000 to 7500
I have only once seen one get all the way up to 8000
 
tdscanuck
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RE: 787 Cabin Pressure

Wed Sep 21, 2011 3:06 am

Quoting bikerthai (Reply 14):
Isn't the relative humidity in the aircraft a red herring in the discussion of corrosion?

No.

Quoting bikerthai (Reply 14):
Even with the current humidity setting in the aircraft, you still get rain in the plane. Humidity, from the passenger breath will still freeze on the fuselage at altitude and melt when the airplanes lands, causing water drain down to the bilge. The problems still exist either way.

The problem magnitude is different. With low relative humidity the rain-in-the-plane problem is reduced because the air contains less water...assuming you're holding the same temperature, lower relative humidity means you've got less water available to condense on the inside of the skin and more water is harmlessly going out the outflow valve.

Quoting KELPkid (Reply 15):
Doesn't the A380 use standard air packs (like just about every other jet transport)?

Yes.

Quoting KELPkid (Reply 15):
I thought the air machine cycle naturally eliminates humidity through the course of its normal operation...

Somewhat, but the ACM has very dry air coming in to start with so that's not really a big factor. The absolute humidity at cruise altitude is functionally nil. Virtually all humidity present comes from the passengers (or humidifiers if so equipped)...the easiest way to regulate humidity is to control the air re-circulation rate.

Quoting tod (Reply 16):
Quoting Chamonix (Thread starter):
The 787 will operate with a higher cabin pressure giving passengers the feeling of being at 6,000 feet above sea level rather than the 8,000 feet for other airliners.

I usually carry an altimeter with me as a pax.

Typical 744 and 777 longhaul flights seem to range 5600 to 6500
Domestic 737, MD80, etc seem to typically run about 7000 to 7500
I have only once seen one get all the way up to 8000

When they say "Aircraft X has cabin altitude of Y" they're really saying "Aircraft X has a *maximum* cabin altitude of Y". Most pressure controllers will use a linear cabin altitude schedule until they hit maximum deltaP, then hold that deltaP up to maximum cruise altitude.

As a result, most of the time, the cabin altitude on any aircraft will be lower than the spec number. However, since they're almost all using linear scaling, a lower max means a lower cabin altitude at any cruise altitude. So a 777 on a longhaul might have 6000' cabin altitude (indicating it's not up at max cruise altitude) but a 787 at the same cruise altitude would have a lower cabin.

Tom.
 
yeelep
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RE: 787 Cabin Pressure

Thu Sep 22, 2011 5:02 pm

Quoting KELPkid (Reply 15):
I thought the air machine cycle naturally eliminates humidity through the course of its normal operation...

The ACM's sole purpose is to reduce air temp. On a 737 the air exits the ACM then travels through a water separator, where the water is removed and the dry air continues through the distribution system.
 
cmf
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RE: 787 Cabin Pressure

Thu Sep 22, 2011 9:54 pm

Quoting KELPkid (Reply 2):
Only if you're using a material subject to corrosion to construct the fuselage   IIRC, there is no metal structure at all in the fuselage...100% composite. There is structural metal on the 787, just not in the fuselage.

It is not just fuselage being affected. Electrical connections are known to create gremlins after interacting with humidity.
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tdscanuck
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RE: 787 Cabin Pressure

Fri Sep 23, 2011 1:45 pm

Quoting yeelep (Reply 18):
On a 737 the air exits the ACM then travels through a water separator, where the water is removed and the dry air continues through the distribution system.

This is pretty common in all ACM designs, since liquid water blobs can wreak havoc on downstream components (especially other ACM wheels). The water load still isn't that high in cruise...fully saturated air at cruise altitude (which would be very unusual) is still less than 5% relative humidity once you bring it up to cabin temperature.

The major water removal happens when the ACM is running on the ground in hot/humid climates.

Tom.
 
baroque
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RE: 787 Cabin Pressure

Sun Sep 25, 2011 3:40 pm

Quoting tdscanuck (Reply 17):
Quoting bikerthai (Reply 14):
Even with the current humidity setting in the aircraft, you still get rain in the plane. Humidity, from the passenger breath will still freeze on the fuselage at altitude and melt when the airplanes lands, causing water drain down to the bilge. The problems still exist either way.

The problem magnitude is different. With low relative humidity the rain-in-the-plane problem is reduced because the air contains less water...assuming you're holding the same temperature, lower relative humidity means you've got less water available to condense on the inside of the skin and more water is harmlessly going out the outflow valve.

No argument there, or

Quoting tdscanuck (Reply 17):
Somewhat, but the ACM has very dry air coming in to start with so that's not really a big factor. The absolute humidity at cruise altitude is functionally nil. Virtually all humidity present comes from the passengers (or humidifiers if so equipped)...the easiest way to regulate humidity is to control the air re-circulation rate.

there. Nor with repisly 20.

Folk do seem a bit odd about RH (not you tds!!!!). In Indonesia, I constantly explain to folk about the need to dehumidify to avoid fungi growing on expensive optics. The common reply is that the A/C takes the water out and they take me outside to the drain to show the water outflow. And then do not listen as I explain that the off-coil air under those circs is guaranteed 100% RH which is just what fungi love!!!!

So good luck with the logic tds!!!

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